DHCP ( Dynamic Host Configuration Protocol )

• A protocol that delivers host-specific configuration parameters from a DHCP server to a host.


• A mechanism for the allocation of network addresses to hosts.

• Automatic allocation

DHCP assigns a permanent IP address to the host.


• Dynamic allocation

DHCP assigns an IP address for a limited period of time. Such a network address is called a lease. This is the only mechanism that allows automatic reuse of addresses that are no longer needed by the host to which it was assigned.


• Manual allocation

The host's address is assigned by a network administrator.

The first service provided by DHCP is to provide persistent storage of network parameters for network clients. The model of DHCP persistent storage is that the DHCP service stores a key-value entry for each client, where the key is some unique identifier (for example, an IP subnet number and a unique identifier within the subnet) and the value contains the configuration parameters for the client.

The second service provided by DHCP is the allocation of temporary or permanent network (IP) addresses to clients. The basic mechanism for the dynamic allocation of network addresses is simple: a client requests the use of an address for some period of time. The allocation mechanism (the collection of DHCP servers) guarantees not to reallocate that address within the requested time and attempts to return the same network address each time the client requests an address.

• Opcode
8 bits.


Value	Description
1	BOOTREQUEST, Boot request.
2	BOOTREPLY, Boot reply




• Hardware type
8 bits.


Value	Description
1	Ethernet.
2	Experimental Ethernet.
3	Amateur Radio AX.25.
4	Proteon ProNET Token Ring.
5	Chaos.
6	IEEE 802.
7	ARCNET.
8	Hyperchannel.
9	Lanstar.
10	Autonet Short Address.
11	LocalTalk.
12	LocalNet (IBM PCNet or SYTEK LocalNET).
13	Ultra link.
14	SMDS.
15	Frame Relay.
16	ATM, Asynchronous Transmission Mode.
17	HDLC.
18	Fibre Channel.
19	ATM, Asynchronous Transmission Mode.
20	Serial Line.
21	ATM, Asynchronous Transmission Mode.
22	MIL-STD-188-220.
23	Metricom.
24	IEEE 1394.1995.
25	MAPOS.
26	Twinaxial.
27	EUI-64.
28	HIPARP.
29	IP and ARP over ISO 7816-3.
30	ARPSec.
31	IPsec tunnel.
32	Infiniband.
33	CAI, TIA-102 Project 25 Common Air Interface.




• Hardware address length

8 bits.


• Hop count

8 bits. This field is used by relay agents. Client sets to zero, optionally used by relay agents when booting via a relay agent.


• Transaction ID

32 bits. A random number chosen by the client, used by the client and server to associate messages and responses between a client and a server.


• Number of seconds

16 bits. The elapsed time in seconds since the client began an address acquisition or renewal process.


• Flags
16 bits


1	16
B	0


B, Broadcast. 1 bit.


• Client IP address

32 bits. Only filled in if client is in BOUND, RENEW or REBINDING state and can respond to ARP requests.


• Your IP address

32 bits. Client IP address


• Server IP address

32 bits. IP address of next server to use in bootstrap; returned in DHCPOFFER, DHCPACK by server.


• Gateway IP address

32 bits. Relay agent IP address, used in booting via a relay agent.


• Client hardware address

16 bytes.


• Server host name

64 bytes.


• Boot filename

128 bytes. Null terminated string; "generic" name or null in DHCPDISCOVER, fully qualified directory-path name in DHCPOFFER.


• BOOTP/DHCP options

Variable length. The first four bytes contain the (decimal) values 99, 130, 83 and 99. This is the same magic cookie as has been defined for BOOTP. The remainder of the field consists of a list of tagged parameters that are called options. All of the vendor extensions used by BOOTP are also DHCP options.

• Allocating a network address

The following summary of the protocol exchanges between clients and servers refers to the DHCP messages described in the following illustrate.


Message	Use
DHCPDISCOVER	Client broadcast to locate available servers.
DHCPOFFER	Server to client in response to DHCPDISCOVER with offer of configuration parameters.
DHCPREQUEST	Client message to servers either (a) requesting offered parameters from one server and implicitly declining offers from all others, (b) confirming correctness of previously allocated address after, e.g., system reboot, or (c) extending the lease on a particular network address.
DHCPACK	Server to client with configuration parameters, including committed network address.
DHCPNAK	Server to client indicating client"s notion of network address is incorrect (e.g., client has moved to new subnet) or client"s lease as expired.
DHCPDECLINE	Client to server indicating network address is already in use.
DHCPRELEASE	Client to server relinquishing network address and canceling remaining lease.
DHCPINFORM	Client to server, asking only for local configuration parameters; client already has externally configured network address.




• Reusing a previously allocated network address

If a client remembers and wishes to reuse a previously allocated network address, a client may choose to omit some of the steps described in the previous section.


• Interpretation and representation of time values

A client acquires a lease for a network address for a fixed period of time (which may be infinite). Throughout the protocol, times are to be represented in units of seconds. The time value of 0xffffffff is reserved to represent "infinity".


• Obtaining parameters with externally configured network address

If a client has obtained a network address through some other means (e.g., manual configuration), it may use a DHCPINFORM request message to obtain other local configuration parameters. Servers receiving a DHCPINFORM message construct a DHCPACK message with any local configuration parameters appropriate for the client without: allocating a new address, checking for an existing binding, filling in 'yiaddr' or including lease time parameters. The servers SHOULD unicast the DHCPACK reply to the address given in the 'ciaddr' field of the DHCPINFORM message.

• Constructing and sending DHCP messages

DHCP clients and servers both construct DHCP messages by filling in fields in the fixed format section of the message and appending tagged data items in the variable length option area.

DHCP uses UDP as its transport protocol. DHCP messages from a client to a server are sent to the 'DHCP server' port (67), and DHCP messages from a server to a client are sent to the 'DHCP client' port (68). A server with multiple network address (e.g., a multi-homed host) MAY use any of its network addresses in outgoing DHCP messages.


• DHCP server administrative controls

DHCP servers are not required to respond to every DHCPDISCOVER and DHCPREQUEST message they receive. For example, a network administrator, to retain stringent control over the clients attached to the network, may choose to configure DHCP servers to respond only to clients that have been previously registered through some external mechanism. The DHCP specification describes only the interactions between clients and servers when the clients and servers choose to interact; it is beyond the scope of the DHCP specification to describe all of the administrative controls that system administrators might want to use. Specific DHCP server implementations may incorporate any controls or policies desired by a network administrator.


• DHCP server behavior

A DHCP server processes incoming DHCP messages from a client based on the current state of the binding for that client. A DHCP server can receive the following messages from a client:

DHCPDISCOVER

DHCPREQUEST

DHCPDECLINE

DHCPRELEASE

DHCPINFORM

■ Parent layer
■ Child layer

BOOTP

DHCP